Should gun manufacturers be allowed to be sued when someone is killed with their product?

Depends. If the one shooting the firearm is killed due to a flaw in the weapon than yes.
If a gun is used in a murder no. You said it yourself-product. By that logic should someone be allowed to sue Ford if I murdered them with my Ranger? The reality is is that if someone has evil in their heart and they truly want to kill someone they will find a way.